The airstrikes come a day after the US and five Arab allies opened their military operation against the Islamic State (IS) terror group in Syria with more than 200 strikes on some two dozen targets.

That campaign, which President Barack Obama has warned could last years, expands upon the aerial campaign the US has already been conducting for more than a month against the extremists in Iraq.

Along with its Arab partners, the Obama administration aims to destroy the Islamic State group, the extremist faction that has through brute force carved out a proto-state in the heart of the Middle East, effectively erasing the border between Iraq and Syria. The United Nations has accused the extremist faction of committing war crimes.

The latest US strikes damaged eight Islamic State vehicles in Syria near the Iraqi border town of Qaim, the US Central Command said in a statement. It also reported hitting two Islamic State group armed vehicles west of Baghdad, as well as two militant fighting positions in northern Iraq.

In a separate statement, Pentagon spokesman John Kirby said the strikes in eastern Syria hit a staging area used by the militants to move equipment across the border into Iraq.

He did not specify exactly where the air raids took place, but the Iraqi town of Qaim is across the border from the Syrian town of Boukamal, where Syrian activists reported at least 13 airstrikes on suspected IS positions today.

The Britain-based Syrian Observatory for Human Rights said it was not immediately clear who carried out the airstrikes in and around Boukamal, but it cited locals as saying the intensity of the air raids was similar to that of strikes on the town early yesterday by the US-led military coalition.

The US-led campaign against the extremists has drawn a mixed response from Syria's multitude of rebel brigades, many of whom have been locked in a deadly fight with Islamic State militants since January.

But the rebels' ultimate goal is to topple Syrian President Bashar Assad, while the US is focused on defeating the Islamic State group.

Today, the main Western-backed Syrian opposition group criticized the American-led airstrikes for being limited to the Islamic State group and other extremists while leaving Assad's government untouched.